Concern regarding the use of variola and monkeypox viruses as bioterrorist agents has led to an increased study of orthopoxviruses to understand the molecular and cellular basis of pathogenesis and develop safe and effective antivirals and vaccines against smallpox. Crucial to these efforts is the availability of animal models, which are inexpensive, genetically homogeneous, and recapitulate the human disease. The popular small-animal orthopoxvirus models employ the inbred mouse as the host, the respiratory tract as the site of virus inoculation, and orthopoxviruses-vaccinia, cowpox, and ectromelia viruses-as surrogates for variola virus. Ectromelia virus is likely the best surrogate for variola virus in a mouse model, as it is infectious at very low doses of virus, and the mousepox disease is associated with high mortality in the susceptible A, BALB/c, and DBA/2 stains of mice, but causes an unapparent infection in the C57BL/6 mouse strain. This chapter describes an ectromelia virus respiratory infection model in the mouse.